fossils and extinction

• Fossils are remains of long-dead organisms left behind, such as skeletons made of rock impressions or entire organisms that haven't decayed yet
• Studying fossils is important because most organisms that have ever lived are now extinct, and fossils provide evidence for evolution by showing incremental changes over millions of years
• Three main ways fossils form:

  • Gradual replacement by minerals: body parts decay slowly and are replaced by minerals, forming rock-like substances in the same shape and size as the original structures
  • Casts: formed when an organism decays in soft material like clay, leaving a gap the same size and shape as the organism
  • Impressions: like footprints or marks left by organisms on the ground over time
• Fossils can also form by preservation where no decay happens, such as in amber, tar pits, glaciers, or peat bogs
• Oldest fossils found are between three and a half to four billion years old, but there are gaps in the fossil record and uncertainties about how life first developed on Earth
• Extinction occurs when no individuals of a species remain, and reasons for extinction include environmental changes, human hunting, new predators, diseases, competition with other species, or catastrophic events like asteroid impacts